Add flow types to Maps in ReactNativeViewConfigRegistry.js (#26064)

Need to add types to these two maps to unblock React Native sync.

scripts/rollup/shims/react-native/ReactNativeViewConfigRegistry.js

@@ -34,8 +34,8 @@ const customDirectEventTypes: {
 exports.customBubblingEventTypes = customBubblingEventTypes;
 exports.customDirectEventTypes = customDirectEventTypes;
 
-const viewConfigCallbacks = new Map();
-const viewConfigs = new Map();
+const viewConfigCallbacks = new Map<string, ?() => ViewConfig>();
+const viewConfigs = new Map<string, ViewConfig>();
 
 function processEventTypes(viewConfig: ViewConfig): void {
   const {bubblingEventTypes, directEventTypes} = viewConfig;